As one of measures to improve the fuel efficiencies of automobiles so as to solve global environmental issues, reduction in body weight has been promoted. For this purpose, steel sheets having high-strengths as much as possible should be produced for use in automobiles. To produce such high-strength parts, there have been employed more and more techniques, in which a steel sheet (blank) is heated into the austenitic region, formed or shaped in a stamping die while being quenched, and thereby yields high-strength parts. Techniques of this type are roughly classified as (1) a direct technique in which the blank is heated to the austenite region, hot-formed the blank in the die, and quenched as intact in the die; and (2) an indirect technique in which the blank is cold-formed without heating, and the formed article is heated and then quenched in the die.
The direct technique is limited in shapes of the parts, because the blank heated to the austenitic region of 900° C. or higher should be formed into a final shape in one stamping process. In addition, for providing registration holes which require satisfactory dimensional accuracy, drilling should be performed after forming. However, drilling in this technique should be performed on a high-strength steel after quenching and thereby requires laser cutting or another technique which is higher in cost than shearing is.
In contrast, the indirect technique requires cold working and die quenching to be performed separately and thereby requires dies in a larger number, but the technique, when employing a steel sheet excellent cold workability, can advantageously give parts having more complicated shapes than those in the direct technique.
Hot-clip galvanized steel sheets are applied as steel sheets for stamping, from the viewpoints of corrosion resistance of the parts and prevention of the generation of oxidized scale on the steel sheets during heating of the parts. Such hot-dip galvanized steel sheets to be applied to the indirect technique are often applied to parts having complicated shapes and should have large elongation and satisfactory stretch flangeability. In addition, the hot-dip galvanized steel sheets should be free from problems such as peeling of the galvanized layer and powdering upon stamping and should be free from surface defects such as tinplating.
As such techniques, for example, Patent Document 1 to 3 disclose methods for producing parts, which employ galvanized steel sheets and ensure high strengths of the steel sheets after quenching. According to these techniques, the steel sheets to be used are designed to have chemical compositions added with various alloy elements (e.g., Si, Cr, Mn, Ti, and B) so as to ensure the strengths of the parts. These added elements, however, may form oxides on the surface of the material steel sheets (surface of the steel sheets) before dipping in a molten zinc pot in a continuous hot-dip galvanization line and may suffer from an unplated area on the steel sheet surfaces.
To solve this problem, there is an applied technique of performing a oxidization-reduction process and subsequently performing plating. In the oxidization-reduction process, the steel sheet surface is once oxidized to form an iron oxide, and the iron oxide is reduced. However, this technique requires dedicated facilities and needs precise control and regulation of oxidization and reduction conditions therein. If the control of these conditions is unsuitable, even the oxidization-reduction may cause an oxide of the alloy element to partially remain on the surface layer of the base steel sheet before plating, and this causes unplating. The galvanized coating reacts with the base iron to form an alloy upon heating after parts forming. The oxide, when remains on the surface layer of base iron, may cause unevenness in alloying speed.
The unevenness in alloying speed upon part heating causes unevenness in surface emittance, and subsequently causes unevenness in temperature upon heating. The unevenness in temperature upon heating in turn causes unevenness in amount of zinc oxide in the surface layer, and this causes unevenness in coating adhesion and weldability.
A steel sheet, when quenched through water quenching (at a cooling rate of several hundred degrees Celsius per second) or quenched in a special die having an enhanced severity of quenching, can have a high strength because of extremely high cooling rate. However, this technique still requires the addition of alloy elements at a certain level or more, because the stamped article, if having a complicated shape as parts in the indirect technique, may not undergo quenching at a sufficiently high cooling rate in all.
The less alloy elements contained the more advantageous for ensuring cold workability and for preventing imperfect hot-dip galvanization. However, certain amounts of alloy elements should be added so as to ensure stable strengths of the parts after die quenching even when the parts have complicated shapes. Thus, demands are made to give a technique which meets both of these requirements.